Electric power systems planners have long accepted high voltage direct current (HVDC) as an attractive alternative to conventional three-phase high voltage AC in certain circumstances. HVDC lines are less expensive than AC lines. The classical economic case for HVDC is made when the savings in line costs are sufficient to offset the rectifier and inverter stations to convert AC to HVDC at the sending end and back to AC at the receiving end. Since HVDC terminals are expensive, applications usually involve longer than normal distances. The economic break-even distance varies greatly from one situation to another but may be in the order of 500 to 800 kM for overhead and one tenth that for underground or under water cables.
There are other advantages to HVDC; primarily the fact that the power transfer can be easily controlled and that short circuit current is much lower; an especially important consideration for metropolitan systems. HVDC is also more efficient in that it uses the insulating strength of the line or cable continuously rather than only during crest voltage as with AC. Thus for the same level of insulation, continuous DC voltage can be at least √2 times the rms AC voltage, power transfer being increased by the same ratio. The increase in DC voltage can be even greater than that since HVDC systems do not require the same additional margin for over-voltages which occur at switching.
The resistance of conductors is also slightly lower for DC current inasmuch as electric fields associated with power frequency AC current forces the current distribution to favor the outer periphery of a conductor. With DC or very low frequency AC, the current distribution is more uniform so the electrical resistance is less.
The above intrinsic characteristics can result in an HVDC conductor transmitting on the order of 60% more power than the same conductor and same insulation in an AC system. However, it's important to note that the power which flows in an AC line may fall far short of it's intrinsic thermal capability, being limited by broader system concerns such as stability, voltage control, reactive power transport, and the fact that actual flow is determined by the system context rather than by controls. For example, a line which is paralleled by several much higher capacity lines may carry very little load in spite of its ability to do so.
HVDC has historically and naturally evolved around a transmission system which uses one conductor for positive voltage and current and another for negative voltage and the equal and opposite return current. This is referred to as bi-pole transmission. Each pole, positive and negative, is equipped with its own mono-pole bridge. The thyrister configuration within a conventional bridge is shown in FIG. 2A. Under normal operating conditions all of the return current is in the second pole, none in the ground.
Some lines have been built with one pole only, in which all return current flows in the ground or a metallic ground wire. In most bipolar lines a separate conductor (or pair of them) is provided so that when one pole is forced out of service, the other can continue to operate. In that state the line operates at half power.